Metallic layers, particularly so-called thin metallic layers on metallic bodies, are made by a number of different chemical and physical methods. So-called thin layers are layers in the nanometer or micrometer range. Thin layers within the meaning of the invention are preferably such layer thicknesses that are smaller than 10 μm, preferably in the range of 0.5 to 10 μm. Depending on the composition of the surface configuration, such layers have, for example, esthetic or technical functions; for example, they can serve as a shield that protects the core material.
The manufacture of thin metallic layers on comparatively thick metallic strips (with a layer thickness ratio of 1:10 and greater) is basically known in the art. Whenever thin metallic surface layers are applied to substrate strips, the prior art provides, however, for them to be generated individually and already present as thin layers, particularly such that they already have the desired end thickness when the layer is made.
Known methods for generating thin metallic surface layers are, for example,
a) galvanizing
b) currentless (chemical) coating
c) hot-dip coating
d) CVD/PVD.
These methods differ in the thickness of the layers that can be generated, as well as in the structure and the properties of the finished layers.
Disadvantageously, these coating methods require that the coating be applied to substrates that are already close to having their final dimensions. This why correspondingly large surface areas must be made, cleaned, activated and coated. Coating methods of this kind are therefore complex and expensive.
Furthermore, layers made by thin-film coating methods, particularly galvanization, are not free of pores. Thinly coated metal strips, particularly when coated by galvanization, must therefore be post-rolled in an additional work step. Along the walls of the pores, it is easier for the core materials, meaning the material of the metal strip that carries the coating, to diffuse through the top layer. To obtain a closed layer, minimum thicknesses are necessary, depending on the deposited material. For a galvanic coating with nickel, this minimum thickness is about 3 μm.
The metal strip that is intended for a coating process has a continuous oxide layer if the oxides are not removed or reduced, or if the coating procedure is not done under a protective gas atmosphere or in a vacuum. The oxide layer creates a contact resistance between this metal strip that is to be coated and the layer to be applied. If the coated metal strip is used for conducting current, this contact resistance must be overcome.
The layers that are made by thin-layer coating methods, particularly by galvanization, lack sufficient adhesive strength to withstand any major following deformation. Thinly coated, particularly galvanically coated, metal strips must therefore undergo diffusion annealing to reinforce the bond. This is especially unavoidable if the laminate must be deformed afterward, for example in a deep-drawing step. During annealing, the oxide layer as such remains intact; however, undesired mixed crystals typically form inside a diffusion layer that is generated between the metal strip that is to be coated and the applied layer. Such a mixed-crystal layer that is formed by diffusion annealing consumes material of the deposited layer and thereby reduces the protective function thereof.